geom_label: Use provider aliasing to alias upstream geoms

For synthetic aliases (just pseudonyms inferred from metadata like GPT or
UFS labels, GPT UUIDs, etc), use the GEOM provider aliasing system to create
a symlink to the real device instead of creating an independent device.
This makes it more clear which labels and devices correspond, and we can
safely have multiple labels to a single device accessed at once.

The confusingly named geom_label on-disk construct continues to behave
identically to how it did before.

This requires teaching GEOM's provider aliasing about the possibility
that aliases might be added later in time, and GEOM's devfs interaction
layer not to worry about existing aliases during retaste.

Discussed with: imp
Relnotes: sure, if we don't end up reverting it
Differential Revision: https://reviews.freebsd.org/D24968